FIP is a statistic designed to measure a pitchers performance, based only on the things that he has direct control.  The term actually stands for Fielding Independent Pitching, the idea being that if you can eliminate the effects of the defense and other factors beyond the pitchers control, you can properly measure a pitchers true value.  The statistic is scaled to ERA…

Wait, what is ERA?

ERA stands or Earned Run Average, which is exactly what it says it is, the average number of earned runs a pitcher gives up per 9 innings.  And before you ask, an earned run is any run given up by the pitcher except for those caused by errors.  ERA is the most common statistic used to measure pitchers.  The formula is simply the number of earned runs allowed divided by the number of innings pitched times 9, (ER/IP)*9.  So if you allow three earned runs in 6 innings, your ERA is 4.50.

OK, back to FIP.  It’s scaled to ERA for two reasons.  The first is so that you can compare a pitchers ERA and his FIP.  If his FIP is lower than his ERA, its likely because he had a good defense playing behind him, though there can be other reasons.  Conversely, if FIP is higher than ERA, the opposite can be concluded.  Sometimes people will say a pitcher was “lucky” or “unlucky” based on a comparison of ERA and FIP, though I personally don’t agree with that assessment.

The second reason FIP is scaled to ERA is to provide an easy frame of reference for fans and analysts.  Since ERA is so common, people already know what is good and bad.  An ERA below 3.50 or so is considered, while an ERA above 5.00 is considered bad, etc.  This prior knowledge is automatically transferred to FIP, without having to learn a new scale in order to evaluate what the numbers mean.

So how do you calculate FIP?

There a 6 variables involved in FIP.  Home Runs (HR), Walks (BB), Innings Pitched (IP), Strike Outs (K), Hit Batsman (HBP), and Intentional Walks (IBB).  The Formula is as follows:

(HR*13+(BB+HBP-IBB)*3-K*2)/IP

A league and year specific Scaling factor is then added so that the average FIP matches the average ERA for a given season.  Generally speaking, the scaling factor is very close to 3.2, and when doing home calculations, its usually safe to use 3.2.

Where can i find more info?

A good place to start is this fangraphs post explaining FIP as it relates to pitcher win values.  These is also this wikipedia article that explains the origins of defense indpentant pitching statistics.  I’m sure there are also lost of other good source that can be found via Google.